Educators note that the quality of the hours students spend in class is as important as the quantity.

"What is 'blaring?'" Emilio Sanchez asks his class of fourth graders during a reading lesson at Vaughn Next Century Learning Center. The children make a sound like a car horn and later discuss many other words they're about to encounter.

Students, and their teachers, have more time to work on such exercises because the San Fernando charter school operates for 195 days, 20 more than the state-mandated 175.

Sanchez and school director Anita Zepeda say the expanded calendar has brought improved test scores, better attendance and higher graduation rates for the 2,400 students in kindergarten through 12th grade.

Zepeda acknowledged that it's a long school year, but supports it. "Our test scores have moved up every year," she said. " We're not saying they're where we want them to be, but every year we've shown growth. "

President Obama, U.S. Secretary of Education Arne Duncan and school districts nationwide have embraced extending the school year. In Los Angeles, the Catholic archdiocese recently announced a goal of moving most of its 210 elementary and middle schools to a 200-day calendar.

Some supporters say U.S. students aren't in school enough, particularly compared to their peers in other industrialized nations, and that both students and teachers benefit from having more time. Others say the expanded calendar, with the same teachers and curriculum, doesn't automatically mean a jump in test scores.

Education research varies over whether extending the school year leads to increased student achievement.

In Massachusetts, where 19 public schools serving more than 10,500 students each added about 300 hours to the school schedule, some students showed greater academic gains than the state averages, although the results were not consistent.

In Miami-Dade County, a pilot program that extended the school day and year at low-performing schools produced mixed results, according to a 2009 evaluation. Some staff members reported fatigue and some students failed to attend summer classes.

Proponents, such as the Boston-based National Center on Time & Learning, argue that an extended calendar can benefit low-income students who lack access to the academic, cultural and social activities of their higher-income peers. The center is a partner in the Massachusetts project. However, a one-size-fits-all approach doesn't work, said the group's president, Jennifer Davis.

"Higher-income communities want expanded time to focus on foreign languages, more arts and cultural opportunities, where high-need students might want more academic time," Davis said. "Policy and money should focus on those high-need students, but students in a higher socioeconomic status can also benefit."

Many advocates of an extended calendar cite the research of Johns Hopkins sociology professor Karl Alexander, who found that low-income children fall behind academically over the summer months and that this accounts for most of their subsequent achievement gap.

But Alexander said that simply keeping students in school longer is not the key.

"If what we want to do is help our neediest children get ahead following the path forged by children from more privileged families, we need to find opportunities for them to do the things that privileged families do, not just 'drill and kill' but solid academic programs combined with an engaging array of enrichment experiences," he said.

Stretching calendars at under-performing schools with poor leadership and poor teachers is not likely to improve student success, said Elena Silva, senior policy analyst at the Washington-based Education Sector.

"So the question about why we should extend school for those kids is a question about whether we can deliver on quality," Silva said. "If you can't ensure that learning is happening, why should we do it?"

With many schools struggling financially, adding days can be too costly. Many schools in the Los Angeles archdiocese will have to raise tuition.

Vaughn, the San Fernando charter, reduced its calendar to 195 days this year, from 200 earlier, because of budget constraints. And the Los Angeles Unified School District has shaved days off the calendar to cope with its budget crises.

Leonidas Macias said the extended calendar at Vaughn has provided academic and extracurricular benefits for her children, who are in the fourth, ninth and 12th grades.

"In our community, there are so many bad things like gangs that tempt students," said Macias, who also works at the school. "People say the school year is too long, but they are learning in a safe environment."